Randy says:
 
 
<The smaller the proportional effect of the treatment, the larger  an "n"
(number of colonies in the trial) you need.>

 
A useful statistic that anyone with an Excel spreadsheet can produce  is 
called 'coefficient of variation' or 'relative standard deviation'.  In  
general, biologists tend to cite CV values, chemists like RSD.  No matter,  its 
the same statistic.
 
To calculate CV or RSD, enter your values, calculate the mean and standard  
deviation for the data set, divide the SD by the Mean and multiply by 100.  
 This gives you a ratio expressed as a percent.
 
Since this ratio is a relative ratio, the units of measurement are not  
important.  The lowest possible value is 0, the highest is usually 100  -  
although for very noisy data, the SD may exceed the mean, and the CV or  RSD can 
be higher than 100.  
 
 
Now that you have your CV or RSD - however you want to call it, the  
interpretation is  easy.  The smaller the ratio, the less noisy  (variable) the 
measurement.  Higher values are noisy.  The higher the  CV or RSD, the more 
colonies you are going  to need for a valid test.
 
 
As an example, when analyzing for inorganic elements like copper, zinc,  
lead, etc. - these metals occur in bees at ppm concentrations - for example,  
80-100 ppm zinc is normal.  The analytical instrument can easily detect ppm  
concentrations.  So, when I set up standards and run them through the  
instrument, I expect to see the instrument output values that are very close to  
the standard, and the CV or RSD may be as low as 1.  If its above 5%, I go  
back and re-calibrate the instrument.
 
Now, if I'm looking for pesticides in bees or pollen, I've a very complex  
matrix, and I'm interested in looking at concentrations of pesticides as low 
as  1 ppb.  That is far more difficult to accomplish, and the GC/MS 
instrument  is not likely to be able to match a standard at 1% or even 5%.  As long 
as  its below 15%, most labs will proceed.  That's why EPA Superfund labs 
often  run triplicate analyses - analyze the same sample three times, and 
take an  average to get the reported result.  That 10-15% CV or RSD is 
unavoidable,  so don't depend on one analysis result - especially if there are 
millions of  dollars in cleanup costs at issue.
 
Now, as per bees.  We've done CV or RSD on multiple colonies for lots  of 
metrics - such as size of bee population, amount of brood, amount of honey,  
etc.
 
Most of the bee population metrics come in at about 25-35%.  Honey  stores 
are more variable, expect 60-70% or more.
 
Toxic chemicals in control colonies (far removed from any source of the  
toxin) may fall below 5%.  Variation in toxic chemicals in areas with  HIGH 
chemical exposure may yield 5-10% - since all of the colonies are  exposed.  
But, if the toxic chemical is patchy - in different nearby  fields, or if a 
plume from an industrial source blows one way today, another  tomorrow, then 
the CV or RSD can exceed 100%.
 
 
One final example - pesticides in soils (or fertilizer, etc.).  These  tend 
to be VERY patchy, and several samples from the same field may yield a CV  
of as high as 200 to 300%.  Getting a fix on the representative value   for 
the field is a huge problem.
 
Jerry
 
 
 
 
 
 



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